1. Field of the Invention
The present invention relates to a motor drive apparatus, and more particularly relates to a motor drive apparatus and a method for reducing electric power consumption while in a stand-by state.
2. Discussion of the Background
In recent years, consciousness of environmental issues has been increasing. For example, demand for reduction in electric power consumption of an electric device, such as a household electrical appliance or an office machine, is increasing. In a lifetime of some electric devices, an aggregated amount of electric power consumption in a stand-by state exceeds the aggregated amount of electric power consumption in an operating state. Therefore, reduction in electric power consumption in the stand-by state of the electric device is important.
When an electric device including a motor is operating, a motor drive apparatus starts and stops the rotation of the motor, and controls the rotation speed thereof according to input signals. Many of these electric devices for convenience are desired to start an operation as soon as an operating command is input by an input device, such as a start button. Therefore, various devices installed in an electric device are supplied with electric power at all times, including when the electric product is in a stand by state, so that the electric device can swiftly start an operation when the start button is depressed.
However, when the electric device is in a stand-by state, the motor drive apparatus structured with semiconductor switching devices and the motor having position sensors are both provided electric power. Consequently, the motor drive apparatus and the motor waste a certain amount of electric power in the stand by state.
FIG. 1 is a schematic circuit diagram illustrating a structure of a motor drive apparatus 100 according to background art and a motor 10 driven by the driving apparatus 100. The motor 10 is a brushless direct current motor having three-phase field windings (hereinafter referred to as a DC motor). In FIG. 1, the DC motor 10 includes a rotor (not shown), Hall-effect devices "H1", "H2" and "H3" denoted by reference numerals 107, 108 and 109, respectively and field windings 110, 111 and 112. For each of the field windings 110, 111 and 112, a phase U winding is denoted with "U", a phase V winding with "V", and a phase W winding with "W". The Hall-effect devices "H1", "H2" and "H3" sense a rotating position of the rotor of the DC motor 10.
The motor drive apparatus 100 includes a power input terminal 101, a grounding input terminal 102, a start and stop signal input terminal 103, a motor control circuit 104, a current switching device 105, a voltage regulator 106, bias resisters 113 and 114 for the Hall-effect devices. The power input terminal 101 inputs electric power having a voltage Vcc relative to the grounding input terminal 102 supplied by a main power source 301 provided in an electric product 300. The voltage regulator 106 inputs the supplied voltage Vcc from an input terminal of the voltage regulator 106 and outputs a voltage Va, which is lower than the voltage Vcc. The voltage Va is supplied to the motor control circuit 104 and the Hall-effect devices 107, 108 and 109 of the DC motor 10 via the bias resisters 113 and 114 as a biasing voltage across the Hall-effect devices 107, 108 and 109.
The motor control circuit 104 includes a start/stop terminal, a speed control circuit (not shown), a position sensing circuit (not shown) for sensing a rotating position of the rotor of the DC motor 10 and a current switching control circuit (not shown). The current switching device 105 connects to the field windings 110, 111 and 112 of the DC motor 10 and the power source having the voltage Vcc. The current switching device 105 also connects to the motor control circuit 104 via resistors each denoted by "R" and transistors each denoted by "Q".
When the electric device 300 is turned on, the main power source 301 is turned on to supply power having voltage Vcc to the voltage regulator 106 and the current switching device 105 of the motor drive apparatus 100. Before the motor control circuit 104 receives a "start signal", i.e., when the electric device 300 is in a stand-by state, the voltage regulator 106 outputs and supplies a voltage Va to the other elements, such as the motor control circuit 104 of the motor drive apparatus 100 and the Hall-effect devices 107, 108 and 109 of the DC motor 10. During the stand-by time, the current switching device 105 does not energize the field windings 110, 111 and 112. However, a current generally flows through the motor control circuit 104, the bias resisters 113 and 114, and the Hall-effect devices 107, 108 and 109, and thereby a certain amount of electric power is consumed. The DC motor 10 and the motor drive apparatus 100 consume, for example, 500 mW to 2000 mW during the stand-by period.
When the motor control circuit 104 receives a "start signal" through the start and stop signal terminal 103, the motor control circuit 104 signals to the current switching device 105 to feed and switch the current running through the field windings 110, 111 and 112 in accordance with a position signal sensed by the Hall-effect devices 107, 108 and 109, such that the field windings 110, 111 and 112 generate a rotating magnetic field and thereby the rotor of the DC motor 10 is rotated.
The motor drive apparatus 100 has a drawback that the motor control circuit 104 is supplied with the voltage Va during the stand-by period, thereby a certain amount of electric power is consumed. In addition, the motor control circuit 104 can be affected by electrical noise, which may lead to a malfunction despite the fact that the electric device 300 is in a stand-by state. Further, during the stand-by period, once the Hall-effect device 107, 108 or 109 fails (e.g., once the device establishes a short-circuit) a large current may run through the current switching device 105 and the DC motor 10, and consequently safety might be impaired in addition to wasting a larger amount of electric power.
In order to decrease electric power consumption in the stand-by state, Japanese Laid-open Patent Publication No. 06006998 describes a brushless motor drive IC and a driving circuit which halt switching elements which drive the motor, except for a circuit for detecting whether the motor is rotating, when the apparatus is in the stand-by state. However, a motor control circuit and Hall-effect devices in the driving circuit are supplied with electric power during the stand-by state and these consume a certain amount of electric power.